In recent years, Internet users have been rapidly increasing. To support increasing communication traffics, optical networks capable of high-speed communications have been widely used. In such optical networks, with an increasing number of nodes for connection and the number of wavelengths, an optical switch using a Micro Electro Mechanical System (MEMS) mirror is used (for example, refer to Japanese Patent Application Laid-open Publication No. 2004-219469).
The configuration of a switch using a MEMS mirror (hereinafter, MEMS optical switch) is explained. FIG. 13 is a drawing of the configuration of a conventional MEMS optical switch. As depicted in FIG. 13, a MEMS optical switch 10 includes an input port 11, an output port 12, an input mirror 13, and an output mirror 14. Other components are similar to those of a known optical switch, and therefore are not exampled herein.
In the MEMS optical switch 10, by controlling the angle of MEMS mirrors placed on the input mirror 13 and the output mirror 14, light input from a channel in the input port 11 is caused to be output from an arbitrary channel in the output port 12.
When it is assumed that the number of channels of the input port 11 is 60 and the number of channels of the output port 12 is 60, the input mirror 13 requires 64 MEMS mirrors and the output mirror 14 requires 64 MEMS mirrors. Therefore, 128 MEMS mirrors in total are required to be controlled.
To control MEMS mirrors, a high voltage (for example, 200 volts [V]) is applied to an electrode of each MEMS mirror in each of vertical and horizontal directions, thereby generating an electrostatic force to control the MEMS mirror (in other words, tilting the MEMS mirror in an arbitrary direction).
Next, a mirror driving circuit that controls MEMS mirrors is explained. FIG. 14 is a functional block diagram of the configuration of a conventional mirror driving circuit. As depicted in FIG. 14, a mirror driving circuit 20 includes a MEMS mirror 21, a connection substrate (Dpram) 22, a computing unit 23, and a high-voltage Digital Analog Converter (DAC) unit 24.
Among these, the MEMS mirror 21 corresponds to the MEMS mirrors placed on the input mirror 13 and the output mirror 14 depicted in FIG. 13. For example, when the number of channels of the input port 11 is 60 and the number of channels of the output port 12 is 60, the MEMS mirror 21 is formed of 128 MEMS mirrors.
The connection substrate (Dpram) 22 is connected to a host device (not depicted in FIG. 14) to control data communications with the host device and to store connection information output from the host device. The connection information indicates a relation between the channels of the input port 11 and the channels of the output port 12 for connection. For example, the connection information includes information indicating that light output from the channel 1 of the input port 11 is input to the channel 2 of the output port 12.
The computing unit 23 computes drive voltages (vertical voltage and horizontal voltage) for driving the MEMS mirror 21 based on the connection information stored in the connection substrate 22, and outputs, as the computation results, information about the drive voltages to the high-voltage DAC unit 24.
Specifically, the computing unit 23 holds a management table, and compares the management table and the connection information to compute drive voltages. FIG. 15 is a drawing of an example of data structure of a conventional management table. As depicted in FIG. 15, the management table has stored therein an input channel, an output channel, an input mirror voltage, and an output mirror voltage in association with each other.
The input channel corresponds to a channel in the input port 11, while the output channel corresponds to a channel in the output port 12. The input mirror voltage indicates voltage (vertical voltage and horizontal voltage) to be applied to a predetermined MEMS mirror placed on the input mirror 13, whilst the output mirror voltage indicates voltage (vertical voltage and horizontal voltage) to be applied to a predetermined MEMS mirror placed on the output mirror 14.
For example, when light input from an input channel “1” is output from an output channel “1”, the input mirror voltage is such that the vertical voltage is “195 volts” and the horizontal voltage is “190 volts”, and the output mirror voltage is such that the vertical voltage is “197 volts” and the horizontal voltage is “187 volts”.
The high-voltage DAC unit 24 applies voltages to the MEMS mirror 21 based on the information about the drive voltages obtained from the computing unit 23 to control the angle of each relevant MEMS mirror of the MEMS mirror 21.
In the conventional technology, however, a plurality of DAC units that can apply a high voltage are required to control each MEMS mirror placed on the input mirror 13 and the output mirror 14. Since such DAC units are extremely expensive compared with general-purpose DAC units, manufacturing cost is disadvantageously high.